1. Field of the Invention
The present invention relates a polymer film. More particularly, the invention relates to a polymer film which causes no smoke generation and no oil contamination at the time of production, is excellent in a surface state and also in trimming and resistance to roll contamination, and can easily control development of optical characteristics. Further, the invention also relates to a phase difference film, a polarizing plate and a liquid crystal display device utilizing the polymer film.
2. Description of the Related Art
Films of polymers represented by cellulose esters, polyesters, polycarbonates, cycloolefin polymers, vinyl polymers, polyimides and the like have been used in silver halide photographic light-sensitive materials, phase difference films, polarizing plates and image display devices. Films more excellent in terms of planarity and uniformity can be produced from these polymers, so that they have been widely employed as films for optical applications. For example, a cellulose ester film having a proper moisture permeability can be directly laminated online to a most commonly used polarizing film comprising polyvinyl alcohol (PVA) and iodine. For this reason, a cellulose acylate, particularly cellulose acetate, has been widely used as protective films for the polarizing plates.
When transparent polymer films are used for the optical applications such as the phase difference films, supports for the phase difference films, the protective films for the polarizing plates and the image display devices, control of their optical anisotropy becomes an extremely important factor to determine the performance (for example, visibility) of display devices.
On the other hand, as methods for producing the transparent polymer films, there have been used solution film forming methods which give a good surface state, or in recent years, there have also been utilized melt film forming methods. In the case of the solution film forming methods, plasticizers are preferably added in producing the polymer films, for the purpose of imparting high-speed film formability. This is because solvents can be evaporated for a short period of time in drying at the time of the solution film formation by adding the plasticizers, thereby being able to decrease the amount of residual solvents in the polymer films. However, in the plasticizer-containing transparent polymer films which have been generally used, undesirable phenomena occur, or the films are adversely affected, in some cases, when the films are tried to be treated under severe conditions in the production process.
For example, when the transparent polymer films are tried to be treated at high temperature, smoke generation occurs, or the films are contaminated with oils, in some cases. For this reason, production conditions or treatment conditions to the plasticizer-containing transparent polymer films have been constrained as a matter of course. On the other hand, it is known that a high-molecular weight plasticizer is used in a photographic triacetyl cellulose ester film. However, it has been difficult to presume that the film is treated at high temperature and can be applied to the optical applications (see JP-A-5-197073).
As an improvement thereof, there is disclosed a technique of adding a plasticizer selected from a polyester and a polyester ether and having a weight average molecular weight of 400 to 5,000 (see JP-A-2002-022956). There is a description that according to this technique, raw material deposition prevention, moisture permeability and size are excellent. However, process contamination at the time of production and raw material volatility at the time of stretching treatment at high temperature are extremely poor, so that this technique can not be practically used. Further, it is disclosed that a specific plasticizer (aromatic ring-containing polyvalent carboxylic acid ester) is used in combination with a polyester (see JP-A-2007-003679). There is a description that according to this technique, a phase difference plate excellent in polarization degree durability and light leakage prevention can be obtained. These improvements are surely observed. However, not only an improvement level by an effect of using the specific plasticizer together appears to be low, but also process contamination by poor volatility of the specific plasticizer in the process is extremely poor. This is therefore lacking productability.